1. Field of the Invention
This invention relates to a method of treating an aqueous solution of crude acrylamide, which has been obtained by catalytic hydration of acrylonitrile and contains cuprous ions, through various steps such as distillation, concentration, filtration, precipitation, active carbon treatment and ion exchange treatment.
2. Description of the Prior Art
As a new commercially attractive method for producing acrylamide, it has been proposed to obtain acrylamide in the form of an aqueous solution directly from acrylonitrile by catalytic hydration (as disclosed, for instance, in U.S. Pat. No. 3,631,104 and Belgian Pat. No. 753,365). The aqueous solution of acrylamide removed from the reactor is marketed in that form or after crystallization. In either case, the product from the reactor is refined or purified while it is an aqueous solution to enhance its commercial value.
The catalytic hydration method is quite different from the sulfuric acid method conventionally used in the prior art for the production of acrylamide wherein a large amount of a sulfate is formed as by-product. Hence, a technique unique to the purification treatment for an aqueous solution of crude acrylamide produced according to the catalytic hydration method is still unknown.
Catalysts utilizable for the aforementioned catalytic hydration method are composed essentially of metallic copper as the effective ingredient. Illustrative of such catalysts are metallic copper catalysts such as Raney copper, reduced copper and Ullmann copper. The catalytic hydration reaction is advantageously carried out in a liquid phase in which case a certain kind of copper salt may be used as co-catalyst in addition to the above-mentioned catalyst (See, for example, Belgian Pat. No. 784,799). In this reaction, water is generally used in an excess proportion to acrylonitrile, for example, in an amount of 50 - 90 % by weight. An adequate solvent may be added to improve the compatibility of acrylonitrile with water. The reaction temperature is preferably within the range of 50.degree.- 200.degree.C, with the range of 50.degree.- 150.degree.C being most preferable for usual cases.
An aqueous solution of crude acrylamide obtained according to the aforementioned method usually contains 1 - 1000 ppm, more generally 5 - 500 ppm of copper ions, coming from the catalyst and co-catalyst, the majority of which exists in the form of cuprous ion. A variety of treatments are required for the crude product, such as distillation or concentration to remove unreacted acrylonitrile, water and solvents, filtration or precipitation to remove catalyst particles or other particles contained in the product, a treatment with active carbon to decolor the aqueous solution, crystallization to obtain crystals and ion-exchange to remove salts containing copper ions.
In this connection, it is a very significant fact that acrylamide is a compound capable of readily undergoing natural polymerization. It will be readily understood that the occurrence of polymerization here would lead to such undesired results as lowering of the purification performance or degradation of the product quality due to contamination with polmerized acrylamide.
It has been well known from an early date that copper ions can serve as stabilizer for acrylamide. Experiments reveal that the cuprous ion is so effective in it stabilizing effect that no other stabilizer is required even under such severe reaction conditions as operations at a temperature exceeding 150.degree.C. Nevertheless, when the aqueous solution of crude acrylamide obtained by that reaction is fed to the purification process as mentioned above, very grave and unexpected polymerization troubles occurred for instance, gelation of the solution within a storage tank due to polymerization or increase in the viscosity of the product solution.
To avoid the polymerization of acrylamide in the aforementioned purification process, it is possible to adopt a method using an effective stabilizer. This method, however, is not practical because it requires a disadvantageous after-treatment of removing the added stabilizer.
Oxygen serves as a very effective polymerization-inhibiting agent for acrylamide. In fact, if the crude product solution is mixed with air by sparing sufficient time, the polymerization of acrylamide during the purification process can be considerably inhibited. However, this method requires the use of special equipment for steadily supplying and mixing oxygen and is complicated. Additionally, insoluble cupric hydroxide is formed by the oxidation of the cuprous ions with oxygen. This compound settles and chokes the columns of granular active carbon, the ion exchange resin column and so forth in the purification process.
Recently, in the manufacture of acrylamide through the reaction of acrylonitrile with water in the presence of a binary reduced copper catalyst a method of obtaining a prolonged life and selectivity of the catalyst by preventing the catalyst from contact with oxygen has been proposed (as disclosed in U.S. Pat. No. 3,642,894). However, this method cannot directly be adopted for the treatment of the aqueous crude acrylamide solution after the hydration reaction as contemplated in the present invention, because the former method and latter treatment are different in the proportion and concentration of the polymerizable substance, the concentration of by-products formed by the reaction, the coexistent active carbon or ion exchange resin and the conditions and purposes of the treatment.